1. Field of the Invention
The invention is a new composition of matter which generally relates to epoxy resin and more particularly to low temperature curing resins. Similar materials have been examined by group art unit 120; however, an examination by Group Art Unit 140 would also be appropriate.
2. Description of the Prior Art
Waveguides, used in the electronics industry, have in the past been fabricated from various metals. While such waveguides have been generally considered to be useful, attempts to improve performance, i.e., achieve a reduction in the cost, weight and thermal expansion coefficients while increasing the dimensional stability of waveguides resulted in the investigation of graphite filled composites as structural materials.
The successful utilization of composites in waveguide structures for space applications imposes several important constraints on the resin system and the cured matrix obtained from it. Relatively low curing temperatures should yield a composite with greatly improved dimensional stability, since such a composite would be expected to contain far less residual stresses due to the differential expansion of the matrix and the reinforcement. From a practical standpoint, the candidate resin system must have a reasonable working life at typical ambient temperatures and also have good handling characteristics when combined with collimated graphite fibers. Cured composites, besides having excellent dimensional stability, should be suitable for use in the space environment. Additionally, the cured resin system should have strength properties calculated to preclude the formation of microcracks during environmental temperature cycling.
Waveguides fabricated from graphite filled epoxy systems have found a measure of success in the industry. The principal manufacture of such devices has been RCA. However, the RCA waveguides exhibit unacceptably high dimensional instability when exposed to a wide temperature range.
While it can be concluded that graphite filled epoxy resin systems have utility in the fabrication of waveguides these prior art systems exhibit certain undesirable properties which diminish their usefulness. Generally, conventional graphite filled epoxy systems must be cured at temperatures of nominally 350.degree. F. The cured structures undergo shrinkage upon cooling which tends to be counter productive to the attainment of a composite having high dimensional stability.
Other prior resin systems either have unacceptably high curing temperatures which result in dimensionally unsuitable composites or insufficient pot life at typical ambient temperatures as in the case of resin systems which cure at low temperatures. The latter systems suffer the additional disadvantage of curing to yield a polymer which has poor retention of strength properties at elevated temperatures.
In a brief literature search for candidate resin systems, one system in particular, a DGEBA resin containing a substituted imidazole as hardener, offered promise. Systems of this type were cited* as giving castings with good strength properties and relatively high Tg after curing at the relatively low temperature of 55.degree. C. Also mentioned in the reference was relatively long pot life of this resin system. We examined this system and found it to be unsuitable for our intended purposes. The disadvantage to the use of the imidazole as a hardener for epoxy resins lies in the difficulty experienced in dissolving it in the resin and as discussed below, the exothermic reaction experienced during cure. FNT *Lee and Neville, Handbook of Epoxy Resins, McGraw-Hill, 1967.
In order to provide a resin which will be useful in the construction of waveguides which possesses none of the aforedescribed disadvantages of prior art resin systems, we have developed a low temperature curing epoxy resin system. Our system fills a definite need for a composite matrix where the ultimate in dimensional stability over a wide temperature range is desirable.